The present invention relates to a temperature self-control type glow plug and, more particu to an improvement in the glow plug disclosed in U.S. Pat. No. 5,132,516.
Glow plugs having various structures have been known as glow plugs used o improve the starting characteristics of diesel engines. The present applicant has also proposed a temperature self-control type glow plug in, e.g., Japanese Patent Laid-Open No. 57-182026, in which an inventive combination of resistive elements made of two types of materials is used to obtain not only a quick heating function but also a temperature saturation function for ensuring stable heating characteristics by preventing overheating of a heating wire.
In a glow plug of this type, a first resistive element serving as a heating element and a second resistive element connected in series with the first resistive element and made of a material having a positive resistance temperature coefficient larger than that of the first resistive element are embedded in a heat resistance electric insulating powder contained in a metal sheath. In addition, a gap is formed between the two resistive elements to delay heat transmission from the first resistive element. With this arrangement, a required high power is supplied to the first resistive element immediately after its energization to quickly generate heat, thus ensuring the quick heating characteristic. In addition, after a predetermined period of time elapses, the power supplied to the first resistive element is decreased with an increase in resistance of the second resistive element due to a temperature rise on the second resistive element side so as to prevent fusing caused by overheating of the first resistive element, thus ensuring the temperature self-saturation function. With such a structure, since an energization circuit for the glow plug need not have a temperature control means for controlling the supply power, the overall cost of a preheating device can be reduced.
Such a conventional glow plug can ensure both the quick heating function and the temperature self-saturation function to a certain degree. It is, however, difficult to realize such a heating characteristic as the heating temperature being decreased in an afterglow period after the engine is started. Although the conventional glow plug can perform an afterglow operation for about several tens seconds, it cannot satisfy a recent demand for a long-time (10 minutes or more) afterglow operation. In order to realize both the quick heating function described above and the function of performing an afterglow operation for a long period of time while decreasing the heating temperature, a relay used in a preheating period and a relay used for an afterglow period must be separately incorporated in the energization circuit for the glow plug, and at the same time, a voltage dropping resistor and the like need to be incorporated in a circuit on the afterglow side. As a result, the number of circuit components is increased to increase the cost of the overall apparatus.
In order to realize such a long-time afterglow operation by using only a glow plug without adding any elements to the circuit, the energization power to a heating element is self-controlled to greatly improve the heating characteristic, thus preventing overheating at a heater portion. In addition, for example, it is required that the glow plug have a temperature self-control function which serves to decrease the saturation temperature to a proper temperature or lower and keep the temperature so as to ensure the durability of a heating wire. Under the circumstances, there is a demand for a glow plug having a heater portion having a quick heating characteristic, a temperature self-saturation characteristic, and the like, and achieving high reliability in terms of heat resistance and the like.
In a glow plug having a sheath heater constituted by a combination of the two types of heating wires described above, in order to realize a quick heating function, a sheath tip in which a front heating wire serving as a heating portion is embedded is formed into a small-diameter portion to have a heat capacity smaller than that of a sheath portion in which a rear heating wire serving as a control portion is embedded. Such glow plugs have been proposed in, e.g., Japanese Patent Laid-Open Nos. 54-60630 and 57-87535. These conventional structures are effective to a certain degree in realizing a quick heating characteristic by supplying a high electric power to the front heating wire at the initial stage of energization to obtain a required heating temperature, but cannot provide a sufficient overshoot function which serves to decrease the heating temperature after the lapse of a predetermined period of time and perform an afterglow operation over a long period of time while ensuring the durability of the heating wire and the like. That is, the conventional structures cannot provide heating characteristics wherein heat is generated first to obtain a required temperature, and the heating temperature is decreased to a sufficiently low temperature with the lapse of time so as to be saturated. There is a demand for some measures to satisfy the above-described requirements by giving careful consideration to these points.
Especially in the above-described glow plug, the sheath heater is formed in such a manner that a heat resistance electric insulating powder is packed in a sheath containing resistive elements, and a swaging process is performed on the sheath from its outer surface side to achieve an increase in density of the sheath, while a required heat transmission and the like are obtained to improve the heating characteristics, and the reliability is ensured. In this case, however, since each component is thin and small, care must be taken to perform proper manufacturing and assembly processes. Therefore, in consideration of these problems in the manufacture, the above-described requirements need to be satisfied.